1. Field of the Invention
This application relates to a method and apparatus for mounting components on a printed circuit board. More particularly, it relates to the use of slots, holes, or cutouts to prevent current from conducting through a printed circuit board when it is heated to a high temperature by a component.
2. General Background
Printed circuit boards are used in a variety of electronic products for interconnecting electronic components and for providing a mounting structure to hold electronic components in place. Printed circuit boards are made from many different materials. The laminate for printed circuit boards are typically produced using phenolic or epoxy resins. Reinforcing materials may include cotton paper, woven glass, mat glass, and glass veil. A popular combination of epoxy resin and glass reinforcement is referred to as FR-4. Other popular combinations of resins and reinforcement are classified by commonly known notations such as FR-2, CEM-1, CEM-3, FR-5 and GI. Common materials used for printed circuit boards are described in Printed Circuits Handbook, third edition by Clyde F. Coombs, Jr. (1988). Most of the base materials for printed circuit boards, however, are designed to operate no more than 85xc2x0 C. to 150xc2x0 C. Some laminates are designed for higher temperatures but are made of ceramics. The disadvantage of ceramic substrates is that they are considerably more expensive than printed circuit boards made of epoxy or phenolic resins.
Electrical components may be assembled on printed circuit boards in a variety of ways. The two main assembly types are through-hole and surface mount. Through-hole components are mounted to the printed circuit boards via connector wire leads or pins, also referred to as mounting legs. Surface mounted components, on the other hand, do not have through-hole wire leads or pins. Surface mounted components are usually mounted to the surface of the printed circuit board by soldering methods such as wave soldering or reflow soldering.
A problem may arise when a component generates heat. This may happen when a component fails or is subjected to an abnormal condition such as an over-voltage condition which may arise during a lifted neutral failure or other event. The heat may conduct down the mounting legs of a through-hole component or down the soldered contacts of a surface mounted component. Heat generated by the component may also dissipate and radiate to the surface of the printed circuit board. If the surface temperature of the printed circuit board is allowed to reach a high temperature, the printed circuit board""s properties begin to change and free electrons allow current to flow between the mounting points of a component.
A component that may generate heat under certain conditions is a Metal Oxide Varistor (xe2x80x9cMOVxe2x80x9d). MOVs may be commercially purchased in several different packages. A common and inexpensive type of MOV is a through-hole component. However, MOVs may also be obtained as a surface mounted component.
MOVs are used in surge suppression and are typically put directly across a line voltage. If the MOV fails, the MOV may generate heat. MOVs may also generate heat when put across a line voltage in series with a fuse. If a fuse is used, the fuse must be large enough to handle thousands of amperes of fast pulsed current to obtain necessary ratings. When a fuse link is large enough to handle the high short-duration pulsed current, there exists a continuous current somewhere between 2.5 amperes and 25 amperes where the fuse will not open. If a MOV is xe2x80x9cprotectedxe2x80x9d by such a fuse and the lower current continuously flows into the MOV, current flows through the MOV and voltage is present across the MOV. This current and voltage at the same time causes a tremendous amount of energy to be released as heat.
As described above, heat dissipating or radiating from a component (such as an MOV) may cause problems if it is allowed to heat the printed circuit board. Accordingly, there is a need for a method and apparatus for mounting components on a printed circuit board to minimize the effects of components that generate heat. The present invention uses slots, holes, or cutout areas to overcome, or at least reduce the effects of, the problems set forth above.
Slots, holes, or cutout areas of printed circuit boards have long been used in the industry for several different reasons. However, they have not been used to solve the problems described above. One use of slots, holes or cutout areas is for physical mounting of through-hole components on printed circuit boards. The lead wires or pins of the components are inserted into the slots, holes or cutouts of the printed circuit board for retention and contact.
Another use of slots, holes, or cutouts is for controlling the natural resonance frequency of a portion of a printed circuit board. This is described in U.S. Pat. No. 5,453,580 to Franke et al. This patent discloses the use of slots, holes or other openings around vibration sensitive circuitry.
Other uses of slots, holes, or cutouts is for voltage isolation such as in the area of horizontal output transistors in monitors and televisions or to help in the isolation of line to load parts of an electronic circuit. Slots, holes, or cutouts may also be used to allow sand, fluid, light or other media to flow from one side to another side. Another use is to cool power resistors via convection.
In accordance with the present invention, a slot, hole or cutout is provided in a printed circuit board between the mounting points of a component. The slot, hole or cutout is sized to prevent the printed circuit board from conducting current after the surface is heated by the mounted component. Further, the slot, hole or cutout is inexpensive and easy to prepare.
In one embodiment of the invention, a printed circuit board assembly comprises of a mounting board having a top surface and a bottom surface. The mounting board has at least two mounting points for receiving an electrical component. The mounting board has a slot between the two mounting points wherein the slot extends from the top surface of the mounting board to the bottom surface of the mounting board. The slot prevents current from passing through the mounting board between each of the mounting points. For a through-hole component, the mounting points are the through-holes of the mounting board where the mounting legs of the component are inserted and soldered. For a surface mounted component, the mounting points are the soldered contacts. The width and length of the slot is sufficiently wide and long to prevent current from passing through the printed circuit board. The slot may have a variety of geometric shapes including circular or square ends or may be rectangular or elliptical in shape.
In another embodiment, a printed circuit board assembly comprises of a mounting board having a top and bottom surface as well as at least two holes for receiving an electrical component. The electrical component has at least two mounting legs and the mounting legs are inserted into the two holes of the mounting board. The mounting board also has a slot between the two holes of the mounting board and extends from the top surface to the bottom surface of the mounting board. The slot prevents current from passing between each of the two holes of the mounting board. The slot is sufficiently wide and long to prevent current from passing through the printed circuit board and the slot may have a variety of geometric shapes including circular or square ends or may be rectangular or elliptical in shape.
In still another embodiment, a printed circuit board assembly comprises of a mounting board and an electrical component that radiates heat to the top surface of the mounting board. The printed board assembly includes a means for attaching the electrical component to the mounting board at least two mounting points. There is also a means for preventing current from passing through the mounting board between each of the mounting points.
In a further embodiment, there is an apparatus for preventing current from passing through a printed circuit board comprising of a circuit board and a heat generating component. The heat generating component has two mounting legs that are attached to the circuit board. The circuit board has a slot between the two mounting legs of the heat generating component.
In yet another embodiment, a printed circuit board assembly includes a mounting board for mounting a plurality of electrical components. The plurality of electrical components includes a heat generating component having at least two mounting legs which are attached to the mounting board. There is an inlet trace and an outlet trace connected to the mounting legs to provide electrical current to the heat generating component. The mounting board has a slot between the two mounting legs and the slot prevents current from passing through the mounting board between the mounting legs of the heat generating component.
In another embodiment, the invention is a method of preventing current in a printed circuit board by machining a slot in the printed circuit board and mounting a component to the top surface of the printed circuit board at least two mounting points. The slot is positioned between the mounting points.
Although the present description uses a MOV as an example electrical component, the present invention is not limited to the mounting of MOVs. The present invention would work for any component that may generate heat sufficient to change the properties of the printed circuit board and to allow current to pass between the mounting points of a component.